Mail sending error 421

[chickens]

Hi,
When trying to send emails, I keep getting a "421 Unexpected failure, please try later" error message and the email won't go out. I had one earlier today that went through eventually after this, but have several queued mails now waiting. (edited to say they've now gone through, so I assume whatever is at issue will resolve itself eventually, but still thought I'd check in about it...)

Mails to my own email address seem to go through just fine.

Any known issues? It started suddenly today. I'm using Foxmail.

Thanks.